Oscillators are common devices used in a wide variety of devices, including radio equipment and modems. Oftentimes, the oscillator must be extremely stable and accurate, but as frequencies of devices increase and as the integration of components in integrated circuits increases, oscillator design and operation become extremely challenging.
For many systems, an oscillator exhibiting an oscillation frequency within a small range of a particular value is required. Pre-screening of oscillator devices and/or of the components in the oscillator device permits selection of an oscillator having as accurate a frequency as is required for the particular application. However, pre-screening is expensive and results in low yields.
Crystal oscillators, while having a high quality factor (Q factor), are not conventionally formed on an integrated circuit. On the other hand, inductor/capacitor (LC) tank circuits, another form of oscillator, have been used to provide an oscillator on an integrated circuit, albeit with a lower Q factor.
One way to improve the precision of the oscillation is to use closed-loop techniques for synchronizing an oscillator frequency and/or phase, either to an input signal such as a data stream or to a predetermined reference oscillator. Perhaps the most prevalent closed-loop technique for synchronizing an oscillator to another signal is with the use of a phase-locked loop (PLL), e.g., as shown in FIG. 1.
In particular, FIG. 1 shows the use of a PLL 400 to sense a frequency and phase of a carrier in an incoming modulated signal. The PLL 400 provides a phase and frequency corrected recovered carrier signal to the phase/frequency detector 102 for comparison with the actually received modulated signal (which due to real world conditions contains noise in the form of phase and frequency variations).
In FIG. 1, a phase/frequency detector 102 receives both the incoming modulated signal on line 420 and the output of the PLL 400 at point 422. The phase/frequency detector 102 compares the phase and frequency of the received modulated signal on line 420 with the phase and frequency generated by the PLL 400 to detect the actual phase and frequency of the carrier frequency as it is received in the receiver. This accurately determined carrier frequency is subtracted from the received modulated signal to result in an output of the recovered information signal.
In more detail, the received modulated signal is input to the PLL 400 at line 420. A band pass filter 408 band pass filters the input modulated signal such that sideband information beyond that desired is eliminated. A phase detector 406, charge pump 404 and loop filter 402 provide a comparative phase for the received modulated signal and the locally sensed carrier frequency, and generate a DC signal for control of a voltage controlled oscillator (VCO) 430. The VCO 430 outputs a particular frequency based on the voltage level of its control input.
While having certain advantages, the requirement of a PLL adds cost and complexity to a circuit, which is of particular concern in lower end applications such as low end cordless telephones or other wireless applications. Moreover, the closed-loop nature of the PLL slows the acquisition time necessary to acquire phase lock with changes in the input signal, and thus may limit the maximum frequency, modulation and overall performance of the receiver in certain applications.
There is thus a need to provide an open-loop oscillator circuit which is capable of being formed in an integrated circuit and which is capable of highly precise and accurate operation.